Recuperating heat from the hot flue gases is not new per se; such systems are found described, for example, in U.S. Pat. Nos. 4,037,567 issued July 26, 1977 to Torres and 4,175,518 issued Nov. 27, 1979 to Reames. In the first of these earlier systems, a recycle loop is provided which extracts heat from waste gases for the heating of a portion of the boiler feed water wherein the water is circulated within the recycle loop without the need to furnish any additional power. In the second of the above patents, a system is provided which, by natural convections, continually recirculates water from the water storage tank through the preheater during periods of times when water is not being drawn off from the hot water heater.